1. Field of the Invention
This invention relates to microfluidic structures. More specifically, it relates to passivating exposed walls of embedded microfluidic structures.
2. Background
Micro electro mechanical systems (MEMS) combine electrical and mechanical functionalities on a single substrate. An example of a MEMS device could be a small mechanical chamber where two liquids (biofluids, drugs, chemicals etc.) are mixed and a sensor interprets the results. MEMS could also be integrated with logic functionalities i.e. having a CMOS circuit to perform some algorithm with the data provided by the sensor. The CMOS circuit could then have circuit elements that transport the results of the algorithm and the sensor input to another device (i.e. output to further devices comprising the overall micro-system).
One of the mechanical processes typically performed by MEMS is transporting small amounts of fluids through channels. These channels are frequently embedded in a covering layer (hereafter called: embedding layer). One of the difficulties in fabricating these microfluidic channels on, for example, a portion of a semiconductor wafer is having fabricated the channel, fabricating an embedding layer over it that does not deposit into the channel, occlude the channel and prevent any fluid from flowing through it.
One method for successfully meeting this challenge is to fabricate the embedding layer over the area where the channel is to be placed, and then fabricating the channel. The embedding layer can be a global layer that covers the entire surface of the wafer, or it can be a local layer that covers only that part of the wafer where the channel is to go as well as a small area around the channel to provide the embedding layer support and attachment to the wafer.
One method of accomplishing this is to locally heat the embedded portion of the substrate (e.g. wafer) where the channel is to go in a reactive atmosphere. The reactive atmosphere combines with the heated substrate forming a gaseous reaction product that can be transported away from the channel in the vapor phase. The energy to heat the substrate locally in this embodiment would have to pass through the embedding layer. Therefore, the embedding layer will have to be transparent to the energy used to heat the substrate material to mill the channel. Another requirement of all such embodiments is that openings need to be available to provide for pathways for the exhaust of the reactant by-products. The engineering of such openings is also disclosed herewith.
One example of a microfluidic channel that may be used in MEMS is the electrokinetic pump. Electrokinetic pumps use an ionic fluid and a current imposed at one end of the channel and collected at the other end of the channel. This current in the ionic fluid impels the ionic fluid towards the collection pad of the electrokinetic pump.
One difficulty with milling the channel after the embedding layer has been deposited is that then the exposed surfaces of the embedded structure are made entirely of the substrate material. In one embodiment, the substrate can be silicon, as is the case in wafers or in integrated circuit flip-chip packages. Exposed surfaces of silicon interact with any ionic fluid that is transported through them. The interaction can take the form of diffusion through the exposed silicon wall or an ionic pressure gradient between the ionic nature of the structure wall and the ionic nature of the fluid. Hence the need for a method for passivating the exposed surfaces, which is also an item disclosed herewith.